This invention relates to a vacuum evaporation method using a sublimable source material for depositing a thin film on a substrate. For example, the method according to the invention is advantageously applicable to the deposition of a light emitting film in a thin-film electroluminescent device.
Vacuum evaporation is one of the prevailing techniques for depositing thin films of metals, semiconductors or dielectrics on various substrates. Either resistance heating or electron-beam heating is employed for heating the evaporant source material in vacuum. The vacuum evaporation technique has merits such as simplicity of the deposition apparatus, high rate of growth of film on the substrate and consequential success in growth to a desired film thickness with little contamination attributed to foreign matter in the vacuum chamber, and ease of forming a desirably patterned film by placement of a suitably apertured mask in front of the substrate surface. Accordingly vacuum evaporation is widely used for forming thin films of, for example, II-VI compound semiconductors such as ZnS, ZnSe, CdS and CaSe to serve as semiconductor layers in thin-film transistors or light emitting layers in thin-film electroluminescent (EL) devices.
As a problem common to deposition of films of II-VI compound semiconductors by vacuum evaporation, the evaporant source material heated in vacuum for vaporization is partly scattered as fine particles, and some of the scattered particles collide against the substrate on which a film is being deposited. As a result, the surface of the deposited film is often studded with particles of several microns and therefore becomes an uneven and rough surface. In the case of forming a thin film of a compound semiconductor such as the light emitting film in a thin-film EL device on an insulator film and then laying the semiconductor film with another insulator film, it is probable that the scattered particles of the source material break through the underlying insulator film and/or the overlaid insulator film to cause break of insulation in several spots (so-called self-healing type breakdown), and in some cases such breaking may grow to the extent of destruction of most of the picture elements (so-called propagation type breakdown).
To solve the above problem, it was proposed (e.g. JP-A No. 57-99723) to place a mesh which allows vapor to pass therethrough but blocks the advance of the scattered solid particles between the substrate in the vacuum chamber and the source material to be evaporated. However, when such a screening mesh is used it becomes difficult to maintain uniformity of thickness of the deposited film as the mesh is gradually clogged with the trapped particles. If the power of the electron-beam generator is raised as a compensating measure for keeping a constant rate of growth of the deposited film, the result is failure to obtain films of uniform characteristics by reason of a change in the evaporating temperature. Besides, there is a limit to the size of particles which the screening mesh can intercept.
In the manufacture of thin-film EL devices by using a vacuum evaporation method it is wished to form a thin film of an electroluminescent phosphor with even surfaces and without pinholes. To meet the wish it is known (JP-A No. 58-157886) to form the phosphor on the substrate by simultaneously but separately evaporating the host material of a II-VI compound semiconductor and an activator. That is, an activator such as Mn, Cu, Ag, Tb or Sm itself is evaporated by heating under vacuum while a sintered body of, for example, ZnS containing no activator is evaporated by electron-beam heating.
However, even by this method it is inevitable that the host material in the form of a sintered body is partly crumbled and scattered in the vacuum chamber as fine particles, some of which arrive at the substrate surface and cause roughening of the surface of the deposited film. This phenomenon is significant when electron-beam heating is employed since crumbling of the sintered body and scattering of the resultant particles are promoted by bumping of gas in the interior of the sintered body and electrification of the crumbled material. This phenomenon becomes serious also when the evaporant source material is a sublimable substance.
In thin-film EL devices the inclusion of the above described fine particles often causes dielectric breakdown of the device or internal separation at the interface between the light emitting layer and an adjacent insulating layer. It is desired that the scattering of fine particles in the vacuum chamber should be extremely suppressed by an easily practicable technique.